A variety of mount assemblies are presently available to isolate vehicle vibrations, such as for automobile and truck engines and transmissions. One of the most popular mounts today is the hydraulicelastomeric mount of the type disclosed in U.S. Pat. No. 4,588,173 to Gold et al., issued May 13, 1986 and entitled "Hydraulic-Elastomeric Mount" (see FIG. 1, marked "Prior Art".
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central orifice in the plate. The first or primary chamber is formed between the partition plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central orifice of the plate and reciprocates in response to the vibrations. The decoupler movement alone accommodates small volume changes in the two chambers. When, for example, the decoupler moves toward the diaphragm, the volume of the primary chamber increases and the volume of the secondary chamber decreases. In this way, at certain small vibratory amplitudes and high frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the large central orifice, an orifice track with a smaller flow passage is provided, extending around the perimeter of the orifice plate. Each end of the track has one opening; one communicating with the primary chamber and the other with the secondary chamber. The orifice track provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler provides at least three distinct dynamic modes of operation. The operating mode is primarily determined by the flow of the fluid between the two chambers.
More specifically, small amplitude vibrating inputs, such as from smooth engine idling or the like, produce no damping due to decoupling. On the other hand, large amplitude vibrating inputs produce high volume, high velocity fluid flow through the orifice track, and accordingly a high level of damping force and smoothing action. The high inertia of the hydraulic fluid passing through the orifice track contributes to the relatively hard mount characteristic in this mode. As a third (intermediate) operational mode of the mount, medium amplitude inputs produce lower velocity fluid flow through the orifice track generally resulting in a medium level of damping. In each instance, as the decoupler moves from one seated position to the other, a relatively limited amount of fluid can bypass the orifice track by moving around the sides of the decoupler to smooth the transition between the operational modes.
Recent developments in hydraulic mount technology have led to the advent of electronic control of the damping characteristics of the mount. Such a hydraulic mount is disclosed in U.S Pat. No. 4,756,513 Carlson et al. issued on July 12, 1988 and entitled "Variable Hydraulic-Elastomeric Mount Assembly", assigned to the assignee of the present invention. This invention represents an improvement over previous mounts in that it provides a variable damping levels in response to sensed vehicle operating conditions. This is accomplished by the use of an inflatable air bladder to selectively control the diaphragm movement from the secondary chamber side of the mount assembly. The inflation of the bladder is directed by an external control circuit and provides different levels of damping. This control circuit includes a series of vehicle mounted transducers communicating with a preprogrammed microprocessor. The transducers supply vehicle/component vibration information to the microprocessor which in turn directs the operation of the bladder. The orifice track sizes/lengths as well as the control circuit are designed to conform to each vehicle application.
It has also recently been suggested to provide additional damping control by regulating movement of the mount assembly by a compressible fluid (air) chamber on the primary chamber side. However, this concept, as illustrated in the German patent publication DE 3447746 Al, published July 7, 1986 (FIG. 5) generally provides only for increased stiffness, especially at high frequencies, since the air chamber is on the outside of the hydraulic chamber. That is, the German designed mount assembly cannot be controlled to allow compression and/or controlled release of the air as an alternative to damping movement of the hydraulic fluid.
While these recently developed mounts are an improvement over the mounts of the prior art, they are thus not without limitations. At higher frequencies (over 20 Hz) the mounts still exhibit relatively high levels of damping and high dynamic rate and thereby provide only relatively hard characteristics. This is due to the described structure of the mounts. At these higher frequencies, relatively large damping fluid flows exhibiting high inertia forces still occur between the hydraulic chambers. Many times during vehicle operation, these high damping levels and dynamic rates are more than desired, resulting in an undesirable hard feel. With the prior art mounts, these high levels are simply not adjustable downwardly in order to obtain less damping and more engine isolation, that is sometimes desirable. Furthermore, the lower dynamic rate and damping especially at the higher frequencies cannot be obtained by such conventional adjustment methods as changing orifice track sizes/lengths and/or decoupler shapes.
A need exists, therefore, for a hydraulic mount assembly providing variable damping levels during vehicle operation, including high frequency conditions. Such a mount would provide desirable operating characteristics throughout the entire range of vehicle operating conditions.